Research Domain

Heat tolerance is defined as the ability of the plant to grow and produce economic yield under high temperature (Wahid et al., 2007). Tolerance to heat comprises of escape or avoidance mechanisms like timing of panicle emergence, spikelet opening during stress period, anther dehiscence. Heat shock proteins are considered to be the stabilizing factors conferring tolerance to heat thereby protection of structural proteins, enzymes and membranes from heat damage is crucial in temperature tolerance. (Maestri et al., 2002).

Plant architecture is an important trait for tolerance to temperature stress. As an example in some genotypes the panicle is surrounded by many leaves the plant will be able to withstand high temperature stress due to increased transpirational cooling and prevention of evaporation from anther due to shading of leaves. The early morning flowering of rice plant is a useful phenomenon imparting heat tolerance to rice plant. These traits can easily be used in breeding programmes as it inherits in simple manner (Yoshida, 1981). Genotypes of Oryza glaberrima flower much earlier in the day with more than 90% of the spikelets reaching anthesis by 09.00 h. (Prasad et al., 2006). This trait can be used for introgression in O. sativa genotypes. It has been reported that cultivars with large anthers are tolerant to high temperatures at the flowering stage. (Matsui and Omasa, 2002).

For high temperature tolerance, traits such as spikelet fertility can be used as a screening tool during the reproductive stage. Selection of heat tolerance should be done for those materials which can tolerate temperatures higher than 38oC. Cultivars such as N22 has already been identified as high-temperature tolerant so these material can be used in breeding programmes as donors. Genetic modification of the male reproductive organs should be emphasized as it is more sensitive to high temperature. Candidates genes can be identified using QTL mapping, by studying the association of the phenotype and its associated markers. Identification and breeding of heat tolerant germplasm should be carried out for exploiting variation both in genotypic and morphological characters.

Most of the rice is currently grown in those areas where the current temperatures are close to optimum. By the end of 21st century the rice yields have been estimated to be reduced by 41% (Ceccarelli et al., 2010). It has been evidenced that increase in night temperature has been the main cause of increase in global mean temperature resulting in decrease yields (Peng et al., 2004 and Sheehy et al., 2005).

Effect of high temperature on rice plant

The optimum temperature for the normal rice development ranges from 27 to 32oC (Yin et al., 1996). Flowering and the booting stage in rice is considered to be most susceptible to temperature. Temperature higher than the optimum induced floret sterility and decreased rice yield (Nakagawa et al., 2003). High temperature causes floret sterility and decreased ability of pollen grains to swell, resulting in poor pollen dehiscence. Temperature increase of 1oC shortened the number of days from sowing to heading in some genotypes. The symptoms of heat stress in rice has been shown in Table 1.

Mechanisms of heat tolerance and Breeding strategies

Heat tolerance is defined as the ability of the plant to grow and produce economic yield under high temperature (Wahid et al., 2007). Tolerance to heat comprises of escape or avoidance mechanisms like timing of panicle emergence, spikelet opening during stress period, anther dehiscence. Heat shock proteins are considered to be the stabilizing factors conferring tolerance to heat thereby protection of structural proteins, enzymes and membranes from heat damage is crucial in temperature tolerance. (Maestri et al., 2002).

Plant architecture is an important trait for tolerance to temperature stress. As an example in some genotypes the panicle is surrounded by many leaves the plant will be able to withstand high temperature stress due to increased transpirational cooling and prevention of evaporation from anther due to shading of leaves. The early morning flowering of rice plant is a useful phenomenon imparting heat tolerance to rice plant. These traits can easily be used in breeding programmes as it inherits in simple manner (Yoshida, 1981). Genotypes of Oryza glaberrima flower much earlier in the day with more than 90% of the spikelets reaching anthesis by 09.00 h. (Prasad et al., 2006). This trait can be used for introgression in O. sativa genotypes. It has been reported that cultivars with large anthers are tolerant to high temperatures at the flowering stage. (Matsui and Omasa, 2002).

For high temperature tolerance, traits such as spikelet fertility can be used as a screening tool during the reproductive stage. Selection of heat tolerance should be done for those materials which can tolerate temperatures higher than 38oC. Cultivars such as N22 has already been identified as high-temperature tolerant so these material can be used in breeding programmes as donors. Genetic modification of the male reproductive organs should be emphasized as it is more sensitive to high temperature. Candidates genes can be identified using QTL mapping, by studying the association of the phenotype and its associated markers. Identification and breeding of heat tolerant germplasm should be carried out for exploiting variation both in genotypic and morphological characters.

Climate change has been a hot topic nowadays and its impact on agriculture and related fields makes the scientific community to work towards innovating new technologies which proves resilient during fluctuations in climate. In a report by IPCC (2001) which states that in the past century the temperature have increased by more than 0.6oC. It is very surprising to know that most of the warming has occurred since the 1970s and also the warmest years has occurred in the past decade. Further, looking at the last 1000 years, the most warmest years have occurred in the last 60 years and this has caused rise in the occurrence of floods and drought (Wassmann and Dobermann, 2007).

As a C3 plant the rise in Co2 concentration will have beneficial effect on rice plant but the overall effect in the tropical areas will be negative. Erratic rainfall and extreme weather events will increase frequencies of both drought and floods. Higher temperature affect the rice crop particularly during the pollination stage which results in more sterile grains and thus less yield. Increase in sea level will cause inundation of more coastal areas and increase in salinity problem of the coastal areas. Change in climate will have effect on insect pest and diseases. Some of the pathogen and insect pest may proliferate and cause epidemics in rice. Drought and floods will cause change in water use efficiency and nutrient use efficiency of the crop and also the nutrient uptake of the rice due to change in the soil microclimate. Rice crop suffers from a number of stresses which hamper the rice production directly or indirectly. Stresses like drought, cold, heat, disease/insect and flooding affects the rice crop economically. It is estimated that the frequency of these stress environment will increase in the near future.

Plant breeding technologies often combine traditional knowledge with cutting edge biotechnological techniques are already making real impact in meting the challenge of climate change. Apart from crop management strategies for climate change Plant Breeding plays a major role in combating this change by evolving such genotypes which can withstand in stress environments. Breeding climate resilient varieties is a comprehensive approach for mitigating the effects of climate change on rice.

The integration of conventional breeding techniques with modern biotechnological approaches which covers the genomics, proteomics and phenomics aspects of the crops makes the breeding process more efficient and evolving the new rice varieties in much shorter time. Genetic resources are a store house for alleles that provide resilience to the crop under various stresses. The traditional cultivars are valuable germplasm which can be used in breeding programmes. MAS for climate resilient traits in rice have proved to be effective in varietal development. QTL mapping for genes conferring resistance to various stresses in rice is quite effective methodology for mapping genes and its introgression in elite varieties. Here we discuss various stresses in rice due to climate change and the breeding strategies for mitigating the stress and development of varieties which will be the future weapon to cope with climate change.

Breeding efforts for yield enhancement have so far been through exploitation of easily accessible yield influencing variability resulting in dwarf plant type and new plant type based high yielding varieties and higher yielding hybrids in such plant type backgrounds, In search and use of yield related variability, efforts have so far been confined to cultivar genepool. It is the perception that needed variability is available in the cultivar genepool for targeted improvement, never prompted breeders to search for and use very large still not uncovered yield genes remaining hidden in the immediate progenitor and distantly related wild species and primitive land races. Convinced of the fact that in the origin, domestication, further differentiation and continued improvement of rice not all the variability had been captured of what originally present, search for still not identified gene sources was initiated using molecular marker technology. During the last 15 years several molecular marker linked yield related novel QTLs have been found to exist in the progenitor species and primitive cultivars by various laboratories in the world including in India. Research efforts are underway at present to stack the harmonious yield enhancing QTLs for exploring the prospects of raising further the genetic yield level. Molecular marker technology aside, possibilities of raising yield level by genetic manipulations involving alien gene sources are also being pursued in all seriousness in the national institutes. Manipulation of plant architecture, biosynthetic pathway of starch etc, are some of the strategies being attempted now.

Varietal technologies of new yield threshold Since the introduction of dwarf high yielding varieties, breeding emphasis has been to sustain the yield gain achieved by progressively improving them with ability to defend themselves against yield destabilizing factors, especially biotic stresses exploiting hostplant resistance. While breeders have succeeded in sustaining the potential of the semidwarf high yielding varieties, progress in enhancing their genetic yield level, further remained disappointing until Chinese breeders succeeded in breaching the genetic yield level of the dwarf through development of commercially feasible hybrid rice technology by late 1970s. The hybrids with about one ton yield advantage over the best varieties, motivated Chinese farmers to plant them extensively to cover over 55% (18 mill.ha) of China’s rice area by mid 1980s and thereby increase rice production by 20 million tonnes annually. Today hybrids are planted over 85% of the rice area there. Convinced of the potential of the hybrid technology and impressed with China’s success story, India revived its earlier abandoned interest to replicate China’s example in 1990 by augmenting hybrid rice research in a network mode supported by the ICAR, the FAO-World Bank and Mahyco. Taking advantage of parental lines ideally adapted to India’s tropical conditions and the experience of China in hybrid breeding, hybrid seed production and hybrid cultivation, India could succeed in the next five years with the release of the first generation hybrids with one ton yield advantage. The achievement earned for India, the distinction of being the second country after China to exploit hybrid rice technology on a commercial scale. With the active involvement right from the beginning, of the private sector seed industry the country could evolve and release as many as 50 hybrids in all maturity groups and grain quality. Their impact sadly, could not however, be felt, because of their disappointingly slow pace of adoption. In 15 year period since its advent, the technology could not cross as yet two million hectares as against the pace at which the technology with similar yield edge could spread to over 18 million hectares in China. The reasons for so slow adoption are its inconsistency in yield performance, less acceptable cooking quality, lack of hybrids of medium late maturity required for over 80% of the area in the wet season and susceptibility to all major pests. Rightly diagnosing the factors constraining wide adoption, many new generation hybrids freed from the deficiencies are now in the pipeline. Their release soon can be hoped to accelerate the pace of adoption of this potential technology in irrigated, mainly irrigated and favourable rainfed shallow lowland ecologies and cover as large as 8 to 10 million hectares by 2015 and thereby add 7-10 million tonnes to the country’s rice production. Simultaneously, breeders have been engaged in designing morph-physiologically still more productive new plant type genotypes encouraged by the recent reports from China of varieties in new plant type background capable of yielding close to three fourths of the theoretical yield of 20 t/ha. The new plant type based on the concept of marriage between genotype and crop geometry is characterized by enhanced biomass with no change in the already increased harvest index (45%) and robust root system. In the development of such super yielding varieties and hybrids, use of subspecific genepools has been found rewarding. Yet another ambitious programme conceived and being launched jointly by the Melinda-Bill Gates Foundation, China and IRRI is Green Super Rice (GSR). The project aims at evolving super yielders combined with high use efficiency of water and nutrients, broad spectrum resistance to all major pests, enriched nutritive quality and adaptation to adverse effects of climate change in varietal and hybrid backgrounds. Several GSR lines now in advanced stages of development, intensive testing and extensive onfarm evaluation in several Asian countries are expected soon to be ready for commercial planting. The GSR germplasm has helped broaden the genetic base for many traits of value enabling countries in the region including India to access and use it along with its own to evolve future varieties of high input use efficiency in the backgrounds of progressively raised genetic yield ceilings.

India has been successful in raising the genetic yield of rice twice through introduction of plant type based high yielding varieties since mid 1960s and exploitation of hybrid vigour since early 1990s. It is sad that on both the occasions, we failed to harness the full potential of the new varietal technologies given the wide yield gap seen between yields achievable in experimental fields and what is actually achieved by farmers. Of the estimated potential yield of 10 t/ha possible in the semidwarf varieties under irrigated ecology, often less than one half of it is only harvested by farmers. Yield gap analysis done at macro level reveals the gap between actual and achievable yields to vary from 30 to 65%. In majority of the rice growing states, the gap is more than 35%. Whereas in Punjab, Tamil Nadu, Haryana and Andhra Pradesh, actual yield has been found to be close to achievable yield, in states like Bihar, Eastern Uttar Pradesh, Orissa etc, the gap is too wide. Narrowing the yield gap is the most potential near-term strategy to raise the production level substantially. Bringing down the gap even by 30 to 40% in the irrigated ecology would help add no less than 20 million tonnes to the nation’s rice production by 2010. The strategy, if extended to the favourable rainfed shallow lowland ecology, would enable addition of another 10 million tonnes. In translating the strategy into so contemplated production advance, what is required is precise diagnosis of factors that contribute to the gap in a given area and correction of them. At macro level, major the factors that contribute to the gap could be inherent nutrient supplying capacity of soil, the level of consumption of fertilizer nutrients and the extent of their distorted use, soil and water quality, extent of adoption of high yielding varieties and pace of spread of new generation varieties, quality seed availability and effectiveness of extension service, while at micro (farmer) level the yield gap is largely due to differences in the level of compliance of the recommenced package of practices. Development of data-base based on extensive survey and analysis to know in order of importance the various checks that contribute to reduction from the highest yield achievable in a given location, would help plan appropriate remedial strategy to narrow down the gap and maximize thereby productivity and production. Of the several extension strategies designed, experimented with and extensively adopted for maximizing the harvestable potential of varietal technologies ‘Integrated Crop Management’ (Improved form of the System of Rice Intensification), designed and promoted by the Food and Agriculture Organization of the United Nations Organization, is an effective one. Fashioned with two broad objectives viz (i) maximization of productivity by narrowing the yield gap and (ii) sustainable production by optimal use of inputs, ICM is site and farmer specific. Strict adoption of the key checks (cultivation practices from seed to grain) identified on the basis of their yield enhancing potential in a given situation is crucial for consolidating the inherent yield potential of the variety concerned. In case of rice, 10 core and optional checks have been identified as crucial. They include use of quality seed, transplanting relatively young and robust (12-15day old) seedlings, wide space planting at 2-3 seedlings per hill, soil stirring 3-4 times at 10 day interval from 15DAT for effective weed management, intermittent irrigation during vegetative phase where practicable, need based nutrient management integrating organic sources where available, integrated pest management with emphasis on need based use of chemical pesticides and timely harvest and post-harvest care. The ICM strategy being practiced across the rice world including in India, has been found to be an effective integrated strategy in narrowing down the yield gap at macro and micro levels as well as in input saving through enhanced use efficiency. It has been confirmed from studies conducted in different countries that ICM economises seed, water and fertilizer respectively by 60, 30 and 40%. Reservation against adoption of this system on account of availability and cost of labour for labour intensive transplanting of young seedlings at recommended spacing and number of seedlings per hill has now been overcome to an extent by mechanizing transplanting. Correction of wide differences in rice productivity between states and districts within states is yet another opportunity and means to add sizeably to rice production. Of the 563 rice growing districts in the country, barring 218, all are of moderately low (1500-2000kg/ha), low (1000-1500kg/ha) and very low (<1000kg/ha) productivity. The 365 (>60%) districts yielding less than the national average are largely in the rainfed eastern, central and western states. They include mainly Assam with 23 out 26 districts with low yields, Bihar (28/38), Madhya Pradesh (40/48) Chattisgarh (16/16), Jharkhand (18/22), Orissa (20/30), Maharashtra (25/32), Eastern Uttar Pradesh (20/31). In most of these states lack of ideal high yielding/improved varieties adapted to abiotic stress conditions like undesirable water regimes characterized by drought and submergence, soil salinity and low fertilizer consumption seem to bring down the productivity. Popularization of high yielding varieties reasonably adapted to such stresses now available, production and supply of quality seed, enhanced fertilizer consumption and correction of soil problems, development of crop life-saving irrigation facilities for drought prone areas are the means at regional level to maximize rice productivity and production.